An electrical bushing is essentially a well-insulated conductor used to transmit power into or out of a wall or other barrier, such as the enclosure of a transformer, circuit breaker, shunt reactor, power capacitor, and other electrical isolation equipment. A primary bushing, as the name suggests, is typically connected to the primary side of a transformer, whereas a secondary bushing is typically connected to the secondary side of the transformer.
The primary bushing is often used in conjunction with a current transformer (“CT”) to monitor the current flowing through the bushing conductor. The current transformers are usually window type transformers, also known as “donut” transformers, that have an opening in the middle of the transformer winding. The opening allows the transformer to be slid over the end of the bushing and onto the main body of the bushing where it may be used to measure the amount of current passing through the bushing conductor. To decrease component cost, a smaller size and lower voltage (e.g., 600V) current transformer may be used, provided the inner diameter of the current transformer is large enough to pass over the end of the bushing.
Bushings are regulated by IEEE STD C37.20.2, which requires that the primary bushings be able to withstand certain voltage and over-voltage conditions. The “pass” criteria under these test conditions dictate a certain minimum direct distance from the conductive part of the bushing to the electrical isolation equipment, referred to as the “strike” distance, and also a minimum linear surface distance from the conductive part of the bushing to the nearest ground plane, referred to as the “tracking” or “creep” distance. While these minimum distances may be relaxed slightly by improving the insulating material around the bushing, the “strike” and “tracking” distances ultimately depend on the voltage levels seen by the electrical isolation equipment to which the bushing is connected.
To satisfy the minimum required tracking distance, existing primary bushings are typically designed with coaxial, radially extending insulation discs or sheds that function to increase the linear surface distance along the length of the bushing. By using large diameter tracking sheds, the actual length of the bushing may be reduced while meeting the minimum required tracking distance. Most bushings are designed with the large diameter tracking sheds located on the side of the bushing proximate to the switchgear, which allows that side of the bushing to have a reduced length compared to the other side of the bushing where the current transformer is mounted. The bushing itself is typically connected to a circuit breaker that performs the switching operations within the switchgear. An annular mounting flange may be coaxially disposed on the bushing for securing the bushing to the housing of the switchgear or other electrical isolation equipment.
The side of the bushing over which the current transformer is mounted usually cannot accommodate any tracking sheds because the inner diameter of a typical low-voltage current transformer is too small for the transformer to pass over the tracking sheds. As a result, that side of the bushing must extend a certain distance past the current transformer mounting area in order to maintain the minimum required tracking distance to the tip of the bushing conductor. This additional distance not only creates a larger footprint for switchgear and similar electrical isolation equipment that require a bushing, but also increases the material cost of the bushing.
Attempts to address the above drawback have met with limited success. One such effort involves using a two-piece bushing design that allows the bushing to be split into two pieces at a point on the main body so the current transformer may be slid onto one of the pieces. The two pieces of the bushing may then be reconnected after the current transformer is slid into place. However, this design requires that there be a break in the bushing conductor, which can lead to higher conductor resistance.
Thus, a need exists for an improved primary bushing for switchgear and similar electrical isolation equipment that addresses the above and other shortcomings of existing primary bushings.